/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.context.support;

import javax.servlet.ServletContext;

import org.springframework.beans.factory.FactoryBean;
import org.springframework.lang.Nullable;
import org.springframework.web.context.ServletContextAware;

/**
 * {@link FactoryBean} that fetches a specific, existing ServletContext attribute.
 * Exposes that ServletContext attribute when used as bean reference,
 * effectively making it available as named Spring bean instance.
 *
 * <p>Intended to link in ServletContext attributes that exist before
 * the startup of the Spring application context. Typically, such
 * attributes will have been put there by third-party web frameworks.
 * In a purely Spring-based web application, no such linking in of
 * ServletContext attributes will be necessary.
 *
 * <p><b>NOTE:</b> As of Spring 3.0, you may also use the "contextAttributes" default
 * bean which is of type Map, and dereference it using an "#{contextAttributes.myKey}"
 * expression to access a specific attribute by name.
 *
 * @author Juergen Hoeller
 * @see org.springframework.web.context.WebApplicationContext#CONTEXT_ATTRIBUTES_BEAN_NAME
 * @see ServletContextParameterFactoryBean
 * @since 1.1.4
 */
public class ServletContextAttributeFactoryBean implements FactoryBean<Object>, ServletContextAware {

    @Nullable
    private String attributeName;

    @Nullable
    private Object attribute;


    /**
     * Set the name of the ServletContext attribute to expose.
     */
    public void setAttributeName(String attributeName) {
        this.attributeName = attributeName;
    }

    @Override
    public void setServletContext(ServletContext servletContext) {
        if (this.attributeName == null) {
            throw new IllegalArgumentException("Property 'attributeName' is required");
        }
        this.attribute = servletContext.getAttribute(this.attributeName);
        if (this.attribute == null) {
            throw new IllegalStateException("No ServletContext attribute '" + this.attributeName + "' found");
        }
    }


    @Override
    @Nullable
    public Object getObject() throws Exception {
        return this.attribute;
    }

    @Override
    public Class<?> getObjectType() {
        return (this.attribute != null ? this.attribute.getClass() : null);
    }

    @Override
    public boolean isSingleton() {
        return true;
    }

}
